 
 (def _ "An alias for the identity function, for providing positional arguments to mapc." identity)
 
-(defn mapc [dataset fs]
-"Takes an array or a hashmap of functions and maps each to the key column for every row."
+(defn mapc
+  "Takes an array or a hashmap of functions and maps each to the key
+  column for every row."
+  [dataset fs]
   (let [fs-hash (if (vector? fs)
                     (zipmap (column-names dataset) fs)
                     fs)

                             (map first))]
    not-found-items))

(defn all-columns [dataset cols]
  "Takes a dataset and any number of integers corresponding to column
  numbers and returns a dataset containing only those columns.



  One advantage of this over using columns is that you can duplicate
  an arbitrary number of columns."
 [dataset cols]
  (let [not-found-items (invalid-column-keys cols dataset)]
    (if (and (empty? not-found-items)
            (some identity cols))
